1. Field of the Invention
The present invention relates to techniques of manufacturing a semiconductor device that has semiconductor thin-film crystals, each having lattice strain. More particularly, the invention relates to a semiconductor device and a method of manufacturing the same by utilizing oxidation and Ge condensation.
2. Description of the Related Art
In recent years, a so-called strained Si-on-insulator (SOI) structure has attracted much attention. The strained SOI structure is a combination of a strained Si structure and a SOI structure comprising buried oxide film and a thin-film Si crystal layer provided on the oxide film. The strained SOI structure comprises a SiGe-on-insulator (SGOI) crystal layer and a strained Si crystal layer. The SGOI crystal layer is a lattice-relaxed layer that is formed on the buried oxide film and about 10 to 200 nm thick. The strained Si crystal layer is provided on the SGOI crystal layer and about 5 to 30 nm thick. Using the strained SOI structure, a field-effect transistor (FET) may be manufactured, whose channel layer is the strained Si crystal layer. The strained Si crystal layer and the SOI structure impart high channel mobility and low capacitance to the FET, respectively. Hence, a semiconductor device of extremely high performance can be provided.
Oxidation and Ge condensation is known as a method of providing an SGOI substrate, on which the strained SOI structure may be fabricated. (See Jpn. Pat. Appln. Publication No. 2002-76347.) In this method, a thin SiGe film having Ge composition of 10% and being about 100 to 500 nm thick is formed on an SOI substrate that comprises a buried oxide film and a Si layer formed on the buried oxide layer and about 30 to 50 nm thick. Then, the resultant structure is subjected to thermal oxidation at high temperature. During the thermal oxidation, Ge atoms diffuse from the SiGe crystal layer into the Si crystal layer. As a result, the SiGe crystal layer and the Si crystal layer fuse together and become thinner. During the thermal oxidation, Ge atoms are rejected from the oxide interface to the SiGe crystal layer, without combining with oxygen atoms. Nonetheless, they do not diffuse into the substrate through the oxide film. Thus, Ge atoms are accumulated in only the crystal layer between the oxide films.
Hence, the Ge concentration increases as the SiGe crystal layer becomes thinner. If a SiGe crystal layer that has Ge composition of 10% and a thickness of 300 nm is formed on an SOI substrate having a Si layer having a thickness of 50 nm and then thermally oxidized in part at 1200° C., forming an oxide film having thickness of 550 nm, an SGOI substrate that has Ge composition of 30% and a thickness of 100 nm will be provided.
As is known in the art, an SGOI substrate manufactured by means of oxidation and Ge condensation has an SGOI layer that has many threading dislocations, due to the behavior the Ge atoms accumulated in the crystal layer interposed between the oxide films and the stress resulting from the behavior of the Ge atoms. As is known, too, plane defect generates in orthogonal directions, due to twin crystals or stacking fault. The threading dislocations or the plane defect imposes adverse influence, such as leakage current, on the characteristics of the FET that has a SI layer formed on the SGOI substrate.